Many air-to-air and air-to-ground, powered and unpowered, guided and unguided munitions have a common feature—fixed, conventionally-shaped airfoil section fins to stabilize and direct the flight path after separation from an aircraft. These weapons, such as the MK80 family of “dumb” bombs and the AIM-9 air-to-air missile, are usually carried by tactical aircraft as external stores, where space for fixed fins is comparatively plentiful, and the range, speed, and radar cross section penalties associated with external carriage are tolerated.
External weapons carriage is a major source of drag and greatly increases radar reflection. Increasing emphasis on stealth technology increases the need for future air-launched weapons, sensors, and cargo of all types to be designed for internal carriage. Folding fin systems are one important approach to diminishing the stowed volume of these internally carried payloads. Internal payload carriage also demands more compact fin configurations. The same technology is similarly useful for any tube or gun launched, guided or unguided projectile or vehicle.
Traditionally, the US military has employed two basic types of folding fin systems. In a first type, airfoil-shaped fins are stowed so that they snap open in a direction parallel to the flight path. In a second type, side-deploying fins wrap around the circumference of the body of the weapon to minimize undeployed volume required for storage during transportation.
The Russian military has deployed several operational ballistic or air-to-air missile systems using an effective fin technology that is different in configuration and operation than any preceding deployable fin system. Termed a “gas dynamic declination device” by the Russians, and a lattice or grid fin in the US, this system consists of several essentially rectangular “paddles” filled with a grid of approximately triangular, square, and diamond-shaped cells formed by a cross-hatching of thin metal. The fins are fixed to the missile body at the root end in a manner that allows them to be folded flat against the body of the missile in storage. Upon launch, the fins are deployed with their broad lattice face perpendicular to the missile body axis, and may be attached to internal mechanisms that allow the fin to be moved for directional control of the payload. Deployment is reliable because air loads on the fin are usually in the direction of desired motion, up and to the rear, although springs or other devices may be used to assure or hasten deployment.
The US has undertaken an extensive evaluation of the lattice or grid fin concept. The first US patent on grid fin technology, U.S. Pat. No. 5,048,773, issued in 1991 and is held by the U.S. Government. There is a Russian patent claim for use of these devices in supersonic powered rockets such as the AA-12. Fulghum, David, “Lattice Fin Design, Key to Small Bombs,” Aviation Week & Space Technology.
Numerous aerodynamic and systems studies, most notably by Mark Miller and David Washington, have been conducted over the past ten years. Miller, M. and Washington, D., “An Experimental Investigation of Grid Fin Design”; Miller, M. and Washington, D. “An Experimental Investigation of Grid Fin Drag Reduction Techniques”; and Miller, M. and Washington, D., “Grid Fins-A New Concept for Missile Stability and Control.” These studies have shown that lattice fins are aerodynamically effective control surfaces that have slightly higher drag than conventional airfoil fins. If increasing priority is given to compact storage, lattice fins have an advantage over conventional systems. They offer interesting secondary advantages as well. They can operate at high angles of attack without flow separation because the multiple channels of the lattice act as guides controlling the flow. Because of their small size and small center-of-pressure travel with large changes of angle of attack, actuator size and power for controllers can be greatly reduced, leaving more space in an air-born system for fuel and other useful payload. Perhaps more importantly for internal carriage, lattice fins allow an air-born payload to maintain similar capability in a smaller package compared to a conventionally finned payload.
The fluid dynamics and performance of lattice fin-equipped bombs, rockets and missiles and other payloads have been extensively studied both analytically and experimentally for a decade. However, the structure of lattice fins has not significantly changed from the steel configuration mentioned in the US Government's 1991 patent on this technology. Operational Russian fins, as well as almost all US experimental lattice fins, have been built from metals to help them resist the high stagnation temperatures of supersonic flight.
Prior art steel lattice fins are expensive to make. These lattice fins are machined from a solid block of metal by electrical discharge machining (EDM) or water jet cutting. Air Force estimates of the cost of a stainless steel lattice fin made by EDM are approximately $2000. This price is beyond the level of reasonableness for many of the more “routine” and expendable classes of the payloads. Thus, a more cost-efficient lattice fin and method for its production are desirable for this technology to transition from a special purpose laboratory curiosity to a widely used performance enhancement.